The present invention relates to an optical pickup device suitable for recording and reproducing data in a digital mode.
FIG. 1 shows the arrangement of a photodetector in a conventional optical pickup device. The photodetector 6 is divided into regions 11, 12, 13 and 14 by three division lines. The outputs of the inner regions 11 and 12 are applied to an addition circuit 15, whereas the outputs of the outer regions 13 and 14 are supplied to an addition circuit 16. The difference between the outputs of the addition circuits 15 and 16 is calculated by a differential amplifier 19. An optical element such as a cylindrical lens imparts astigmatism to the light beam applied to the photodetector. The width (the sum of the widths) of the regions 11 and 12 is determined so that the output of the differential amplifier 19 is zero when the optical system is focused on an object. When the optical system is defocused, the section of the light beam on the photodetector is changed from a circle into a vertically or horizontally elongated ellipse. Therefore, the output of the differential amplifier 19 may be used as a focus error signal according to the so-called "astigmatism principle".
On the other hand, the outputs of the regions 11 and 13 on the left-hand side are applied to an addition circuit 17, whereas the outputs of the regions 12 and 14 on the right-hand side are supplied to an addition circuit 18. The difference between the outputs of the addition circuits 17 and 18 is calculated by a differential amplifier 20. Under the untracked condition, that is, under the condition where the center of a pit formed on the disk is not irradiated by a beam, amounts of light being diffracted by the pit (the portions indicated by oblique lines in the figure) are made different. As a result, the difference between the outputs of the addition circuits 17 and 18 is obtained, which is representative of the tracking error signal is called as "push-pull principle" and is known in the art.
When, in response to the tracking error signal, the objective lens (not shown) is moved in a direction perpendicular to the track, the light beam is moved on the photodetector 6 in a direction S perpendicular to the direction T. As a result, the focus error signal and the tracking error signal include components other than the true error components, and accordingly it is impossible to perform focus control and tracking control operations with high accuracy.